Signal processing circuit of anti-pollution optical encoder

ABSTRACT

There is provided an optical encoder including an encoding medium, multiple photodiodes and a signal processing circuit. The encoding medium includes an index track. The index track includes a first index pattern and a second index pattern. The multiple photodiodes include a first index photodiode, a second index photodiode and a reference index photodiode for receiving modulated light from the first index pattern and the second index pattern. When the first index photodiode is aligned with the first index pattern and the second index photodiode is aligned with the second index pattern, the signal processing circuit outputs a triggering signal.

The present application is a divisional application of U.S. Ser. No.17/104,822, filed on Nov. 25, 2020, the disclosure of which is herebyincorporated by reference herein in its entirety.

To the extent any amendments, characterizations, or other assertionspreviously made (in this or in any related patent applications orpatents, including any parent, sibling, or child) with respect to anyart, prior or otherwise, could be construed as a disclaimer of anysubject matter supported by the present disclosure of this application,Applicant hereby rescinds and retracts such disclaimer. Applicant alsorespectfully submits that any prior art previously considered in anyrelated patent applications or patents, including any parent, sibling,or child, may need to be re-visited.

BACKGROUND 1. Field of the Disclosure

This disclosure generally relates to an optical encoder and, moreparticularly, to an optical encoder that has a higher tolerance to thecontamination on an encoding medium thereof and a signal processingcircuit thereof.

2. Description of the Related Art

The optical encoder generally includes a light source, a code disk andmultiple photodiodes. The code disk has slits for modulating emissionlight of the light source. The photodiodes detect modulated light fromthe code disk to output detected signals each has a phase shift fromanother detected signal. The processor calculates a rotation angle ofthe code disk according to intensity variation of the detected signals.

In the rotary optical encoder, the code disk is further arranged with anindex slit for recognizing an absolute position thereof. Meanwhile, theoptical encoder further includes an index photodiode to exclusivelydetect modulated light from the index slit to identify whether the codedisk is rotated to a predetermined angle. However, in a high pollutionenvironment that has dust and fragments, when the contamination attacheson the code disk and within a sensing range of the index photodiode, thecontamination can reflect or block emission light from the light sourceto cause the index photodiode to output error signals such that aposition misidentification can occur.

Accordingly, the present disclosure further provides an optical encoderand a signal processing circuit thereof that have high noise toleranceto be adapted to the high pollution environment.

SUMMARY

The present disclosure provides an optical encoder and a signalprocessing circuit thereof in which at least two index patterns arearranged in an index track on an encoding medium of the optical encoderand at least three index photodiodes are included. The signal processingcircuit generates a triggering signal according to the AND operationbetween two comparison signals to indicate a predetermined position orangle of the encoding medium.

The present disclosure provides a signal processing circuit of anoptical encoder including a trans-impedance amplifier (TIA), a firstcomparator, a second comparator and an AND gate. The optical encoderincludes a first index photodiode, a second index photodiode and areference photodiode. The TIA is electrically coupled to the first indexphotodiode, the second index photodiode and the reference indexphotodiode, and configured to convert a first photocurrent generated bythe first index photodiode, a second photocurrent generated by thesecond index photodiode and a reference photocurrent generated by thereference index photodiode respectively to a first voltage, a secondvoltage and a reference voltage. The first comparator is configured tocompare the first voltage and the reference voltage to output a firstcomparison voltage. The second comparator is configured to compare thesecond voltage and the reference voltage to output a second comparisonvoltage. The AND gate is configured to receive the first comparisonvoltage and the second comparison voltage to accordingly generate anindex output.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, advantages, and novel features of the present disclosurewill become more apparent from the following detailed description whentaken in conjunction with the accompanying drawings.

FIG. 1 is a schematic diagram of an encoding medium of an opticalencoder according to one embodiment of the present disclosure.

FIG. 2 is a schematic diagram of an optical encoder according to oneembodiment of the present disclosure.

FIGS. 3A to 3E are operational schematic diagrams of an optical encoderaccording to one embodiment of the present disclosure.

FIGS. 4A and 4B are circuit diagrams of a signal processing circuit ofan optical encoder according to some embodiments of the presentdisclosure.

FIG. 5 is a schematic diagram of signals of the signal processingcircuit in FIGS. 4A and 4B.

FIG. 6 is a schematic diagram of an optical encoder according to anotherembodiment of the present disclosure.

FIGS. 7A and 7B are circuit diagrams of a signal processing circuit ofan optical encoder according to other embodiments of the presentdisclosure.

DETAILED DESCRIPTION OF THE EMBODIMENT

It should be noted that, wherever possible, the same reference numberswill be used throughout the drawings to refer to the same or like parts.

The optical encoder of the present disclosure confirms whether anencoding medium is moved or rotated to a predetermined position bycomparing the photocurrent outputted by two sets of photodiodes so as toeliminate incorrect index output triggered by photocurrent noise causedby the contamination on the encoding medium.

Referring to FIG. 1 , it is a schematic diagram of an encoding medium100 of an optical encoder according to one embodiment of the presentdisclosure. The encoding medium 100 is a code disk or a code stripaccording to different applications. The encoding medium 100 is arrangedwith a plurality of slits or a plurality of reflective stripes asencoding patterns for modulating incident light when the encoding medium100 is moving or rotating.

The encoding medium 100 includes an index track and a position track 12arranged at different tracks. The position track 12 is used to indicatea moving distance of the encoding medium 100, e.g., rotation angleherein. The index track is used to indicate a predetermined position ofthe encoding medium 100, e.g., an original point. Accordingly, when theindex pattern of the index track is not detected, the rotation angle iscontinuously accumulated or counted based on the detection of theposition track 12; whereas, when the index pattern of the index track isdetected, the accumulated or counted rotation angle is reset to zero,wherein whether the index pattern is detected or not is described usingan example hereinafter.

For example, FIG. 1 shows that the encoding medium 100 is a code disk,and thus the index track and the position track 12 are at differentradial positions in a radial direction on the encoding medium 100,wherein the radial direction is perpendicular to a moving direction ofthe encoding medium 100, e.g., a rotation direction herein. In thepresent disclosure, the index track includes a first index pattern IdxAand a second index pattern IdxB arranged at the same radius. The opticalencoder further includes multiple light detecting elements, e.g.,photodiodes, arranged corresponding to the index track and the positiontrack 12.

Please referring to FIG. 2 , it is a schematic diagram of an opticalencode according to one embodiment of the present disclosure. Theoptical encoder includes a light source 200 for illuminating lighttoward the encoding medium 100 to allow the index track and the positiontrack 12 to modulate emission light from the light source 20. Since oneobjective of the present disclosure is to improve the noise reductionability of the index signal, FIG. 2 shows only the first index patternIdxA and the second index pattern IdxB as well as the index photodiodesPD_I1+, PD_I2+ and PD_I− corresponding to the index track, but omits theposition track 12 and corresponding photodiodes thereof.

The position track 12 and the corresponding photodiodes thereof may useconventional configurations without particular limitations; for example,referring to the U.S. patent application Ser. No. 16/878,054, filed onMay 19, 2020 and assigned to the same assignee of the presentapplication, and the full disclosure of which is incorporated herein byreference.

That is, the optical encoder of the present disclosure includes multiplephotodiodes for detecting the light variation caused by the relativemovement with respect to the encoding medium 100, i.e., detectingmodulated light.

Corresponding to the index track of the encoding medium 100, the opticalencoder includes a first index photodiode PD_I1+, a second indexphotodiode PD_I2+ and a reference index photodiode PD_I− to respectivelygenerate a first photocurrent I1+, a second photocurrent I2+ and areference photocurrent I-according to the light variation. The indexphotodiodes and the index patterns are arranged in the way that when thefirst index pattern IdxA of the index track is aligned with the firstindex photodiode PD_I1+ according to the relative movement of theencoding medium 100, the second index pattern IdxB is aligned with thesecond index photodiode PD_I2+. In the index track, the reference indexphotodiode PD_I− is between the first index photodiode PD_I1+ and thesecond index photodiode PD_I2+.

Please referring to FIGS. 3A to 3E, they are operational schematicdiagrams when the encoding medium 100 is moving with respect to theindex photodiodes, wherein each of FIGS. 3A to 3E shows a differentoutput combination of index photodiodes to indicate one event (e.g.,shown as events E1 to E5), and the output photocurrent is shown in FIG.5 . It is seen from FIG. 5 that the photocurrents include not onlymaximum values and zero values but also include tilted sections, and thetilted sections are formed according to the variation of overlapped areabetween the index photodiodes and the light pattern of index patterns.

In FIGS. 3A to 3E, the regions shown by IdxA and IdxB respectivelyindicate reflective light pattern or transmissive light patternassociated with the first index pattern IdxA and the second indexpattern IdxB. The index photodiodes PD_I1+, PD_I2+ and PD_I− are shownto have an identical sensing area, but the present disclosure is notlimited thereto. FIGS. 3A to 3E show that the photocurrent is outputtedwhen one index photodiode is overlapped with the light pattern of oneindex pattern. When the index photodiode is aligned with the lightpattern of the associated index pattern, the maximum photocurrent isoutputted. For simplification purposes, the overlap between the lightpattern of one index pattern and one index photodiode is illustratedherein by the overlap between one index pattern and one indexphotodiode. In the present disclosure, one index pattern being alignedwith one index photodiode means a relative position or angle at whichthe index photodiode receives a maximum light energy of reflected lightor transmissive light from the index pattern.

As shown in FIG. 3A, in an event E1, the index pattern starts to have anoverlap with the index photodiode. The second index pattern IdxB isaligned with the first index photodiode PD_I1+. Accordingly, at aposition corresponding to E1 in FIG. 5 , the first photocurrent I1+ haspositive values, whereas the second photocurrent I2+ and the referencephotocurrent I1 are smaller than the first photocurrent I1+.

As shown in FIG. 3B, in an event E2, the second index pattern IdxB isaligned with the reference index photodiode PD_I−. Accordingly, at aposition corresponding to E2 in FIG. 5 , the reference photocurrent I−has positive values, and the first photocurrent I1+ and the secondphotocurrent I2+ are smaller than the reference photocurrent I−.

As shown in FIG. 3C, in an event E3, the second index pattern IdxB isaligned with the second index photodiode PD_I2+ and the first indexpattern IdxA is aligned with the first index photodiode PD_I1+.Accordingly, at a position corresponding to E3 in FIG. 5 , the firstphotocurrent I1+ and the second photocurrent I2+ have positive values,and the reference photocurrent I− is lower than the first photocurrentI1+ and the second photocurrent I2+.

As shown in FIG. 3D, in an event E4, the index pattern starts to leave aregion of the index photodiode. The first index pattern IdxA is alignedwith the reference index photodiode PD_I−. Accordingly, at a positioncorresponding to E4 in FIG. 5 , the reference photocurrent I− haspositive values, and the first photocurrent I1+ and the secondphotocurrent I2+ gradually decrease to zero.

As shown in FIG. 3E, in an event E5, the first index pattern IdxA isaligned with the second index photodiode PD_I2+. Accordingly, at aposition corresponding to E5 in FIG. 5 , the second photocurrent I2+ haspositive values, whereas the first photocurrent I1+ and the referencephotocurrent I− are zero.

It should be mentioned that although FIGS. 3A to 3E show that a width ofthe second index pattern IdxB in a direction of the relative movement islarger than a width of a sensing surface of the index photodiodes andlarger than a width of the first index pattern IdxA, the presentdisclosure is not limited thereto. In other aspects, the first indexpattern IdxA and the second index pattern IdxB of the index track haveidentical widths in the direction of the relative movement.

It is seen from FIG. 3C that when the encoding medium 100 is moved orrotated to a position at which the first index pattern IdxA is alignedwith the first index photodiode PD_I1+ and the second index pattern IdxBis aligned with the second index photodiode PD_I2+, the reference indexphotodiode PD_1− is not aligned with the first index photodiode PD_I1+or the second index photodiode PD_I2+ such that the first photocurrentI1+ and the second photocurrent I2+ are both larger than the referencephotocurrent I− so as to generate an index output PZ by the signalprocessing circuit, as shown in FIG. 5 .

Please referring to FIG. 4A, it is a circuit diagram of a signalprocessing circuit 400 of an optical encoder according to one embodimentof the present disclosure. The signal processing circuit 400 includes atrans-impedance amplifier TIA, a first comparator C1, a secondcomparator C2 and an AND gate. The TIA is electrically coupled to thefirst index photodiode PD_I1+, the second index photodiode PD_I2+ andthe reference index photodiode PD_I− to receive a first photocurrent I1+generated by the first index photodiode PD_I1, a second photocurrent I2+generated by the second index photodiode PD_I2+ and a referencephotocurrent I− generated by the reference index photodiode PD_I−. Thesignal processing circuit 400 converts the first photocurrent I1+, thesecond photocurrent I2+ and the reference photocurrent PD_I−respectively to a first voltage I1+(V), a second voltage I2+(V) and areference voltage I−(V). The first comparator C1 compares the firstvoltage I1+(V) with the reference voltage I−(V) to output a firstcomparison voltage P1(V). The second comparator C2 compares the secondvoltage I2+(V) with the reference voltage I−(V) to output a secondcomparison voltage P2(V). The AND gate receives the first comparisonvoltage P1(V) and the second comparison voltage P2(V) to accordinglygenerate an index output PZ, as shown in FIG. 5 .

Please referring to FIGS. 3A to 3E and FIG. 5 again, when the encodingmedium 100 is moved or rotated to the position as shown in FIG. 3C, thefirst index pattern IdxA is aligned with the first index photodiodePD_I1+ and the second index pattern IdxB is aligned with the secondindex photodiode PD_I2+ such that the first photocurrent I1+ and thesecond photocurrent I2+ are both larger than the reference current I1.Accordingly, the first voltage I1+(V) and the second voltage I2+(V) havea potential change such as a positive pulse, but not limited to, asshown corresponding to the event 3 in FIG. 5 . After the AND gate, anindex output PZ also has a potential change (also show as a positivepulse) to indicate a predetermined position, e.g., the position shown inFIG. 3C, of the encoding medium 100 of the optical encoder.

In the present disclosure, the signal processing circuit 400 is, forexample, coupled to a processor (e.g., a digital signal processor or anapplication specific integrated circuit) or integrated in the processor.As mentioned above, when identifying the potential change of the indexoutput PZ, e.g., as shown in FIG. 5 , the processor resets as angleaccumulated according to the position track 12 and records that theencoding medium 100 rotates one circle.

Please referring to FIG. 4B, it is a circuit diagram of a signalprocessing circuit 400′ of an optical encoder according to anotherembodiment of the present disclosure. The signal processing circuit 400′includes, for example, a first trans-impedance amplifier TIA1 and asecond trans-impedance amplifier TIA2. The TIA1 includes a first inputend and a second input end, wherein the first input end is used toreceive a first photocurrent I1+ to generate a first voltage I1+(V) at afirst output end of TIA1, and the second input end is used to receive areference photocurrent I1− to generate a first reference voltage I1−(V)at a second output end of TIA1. The TIA2 includes a third input end anda fourth input end, wherein the third input end is used to receive asecond photocurrent I2+ to generate a second voltage I2+(V) at a thirdoutput end of TIA2, and the fourth input end is used to receive thereference photocurrent I1− to generate a second reference voltage I2−(V)at a fourth output end of TIA2. As the second input end of the TIA1 andthe fourth input end of the TIA2 receive the same reference current I1−,if the parameter deviation between the TIA1 and TIA2 is ignored, thefirst reference voltage I1−(V) is substantially identical to the secondreference voltage I2−(V).

According to FIG. 5 , in the above embodiment, when the first indexpattern IdxA is aligned with the first index photodiode PD_I1+ and thesecond index pattern IdxB is aligned with the second index photodiodePD_I2+, i.e. P1(V) and P2(V) both having a pulse, the index output PZ ofthe signal processing circuit 400 or 400′ generates a pulse. Because thepulse of index output PZ is not generated by any one of P1(V) and P2(V),the noise reduction ability is increased.

The present disclosure may further improve the noise reduction abilityby increasing the number of the index photodiodes and the indexpatterns. Please referring to FIG. 6 , it is a schematic diagram of anoptical encoder according to another embodiment of the presentdisclosure. In this embodiment, in addition to the first indexphotodiode PD_I1+ and the second index photodiode PD_I2+, the opticalencoder further includes a third index photodiode PD_I3+. In addition tothe first index pattern IdxA and the second index pattern IdxB, theindex track further includes a third index pattern IdxC. The indexphotodiodes and the index patterns are arranged in the way that when theencoding medium 100 is moved or rotated to a position at which the firstindex pattern IdxA is aligned with the first index photodiode PD_I1+ andthe second index pattern IdxB is aligned with the second indexphotodiode PD_I2+, the third index pattern IdxC is aligned with thethird index photodiode PD_I3+, and the reference index photodiode PD_I−is not aligned with the first index pattern IdxA, the second indexpattern IdxB or the third index pattern IdxC. A width of a sensingsurface of the third index photodiode PD_I3+ is identical to or smallerthan a width of the third index pattern IdxC.

Corresponding to the arrangement of FIG. 6 , the TIA of the signalprocessing circuit of this embodiment is further electrically coupled tothe third index photodiode PD_I3+ and used to convert a thirdphotocurrent I3+ generated by the third index photodiode PD_I3+ to athird voltage I3+(V), as shown in FIG. 7A. The signal processing circuitfurther includes a third comparator C3 used to compare the third voltageI3+(V) with the reference voltage I−(V) to output a third comparisonvoltage P3(V). The AND gate receives the first comparison voltage P1(V),the second comparison voltage P2(V) and the third comparison voltageP3(V) to accordingly generate an index output PZ, wherein the method ofgenerating the first comparison voltage P1(V) and the second comparisonvoltage P2(V) has been illustrated above, and thus details thereof arenot repeated herein. Similarly in this embodiment, the index output PZhas a potential change when the first photocurrent I1+, the secondphotocurrent I2+ and the third photocurrent I3+ are all larger than thereference current I− to indicate a predetermined position, i.e. aposition shown in FIG. 6 , of the encoding medium 100 of the opticalencoder.

Please referring to FIG. 7B, in another aspect, the signal processingcircuit includes a first trans-impedance amplifier TIA1, a secondtrans-impedance amplifier TIA2 and a third trans-impedance amplifierTIA3. The TIA1 includes a first input end and a second input end,wherein the first input end is used to receive a first photocurrent I1+to generate a first voltage I1+(V) at a first output end, and the secondinput end is used to receive a reference photocurrent I1− to generate afirst reference voltage I1−(V) at a second output end. The TIA2 includesa third input end and a fourth input end, wherein the third input end isused to receive a second photocurrent I2+ to generate a second voltageI2+(V) at a third output end, and the fourth input end is used toreceive the reference photocurrent I1− to generate a second referencevoltage I2−(V) at a fourth output end. The TIA3 includes a fifth inputend and a sixth input end, wherein the fifth input end is used toreceive a third photocurrent I3+ to generate a third voltage I3+(V) at asixth output end of TIA3, and the sixth input end is used to receive areference photocurrent I1− to generate a sixth reference voltage I3−(V)at a sixth output end of TIA3. Similarly, as the second input end of theTIA1, the fourth input end of the TIA2 and the sixth input end of theTIA3 receive the same reference current I1−, if the parameter deviationbetween the TIA1 to TIA3 is ignored, the first reference voltage I1−(V),the second reference voltage I2−(V) and the third reference voltageI3−(V) are substantially identical to each another.

In other aspects, the encoding medium 100 is arranged with more thanthree index patterns, and the optical encoder includes more than fourindex photodiodes. When the output photocurrent of the reference indexphotodiode is smaller than or larger than the photocurrent of all otherindex photodiodes, the signal processing circuit generates an indexoutput pulse to indicate a predetermined position of the encoding medium100.

It should be mentioned that although FIG. 2 shows a reflective opticalencoder as an example for illustration, the present disclosure is notlimited thereto. When the light source 20 and the light detectingelements are arranged at two different sides of the encoding medium 100,a transmissive optical encoder is formed and the index pattern is formedby index slits for light to penetrate therethrough. A person havingordinary skill in the art would understand the configuration of theencoding medium 100 and the signal processing circuit adapted to thetransmissive optical encoder after understanding the operation of thereflective optical encoder mentioned above.

It should be mentioned that although the index patterns are illustratedby a shape of rectangle in the above embodiment, the present disclosureis not limited thereto. In other aspects, the index patterns have othershapes, e.g., the trapezoid or triangle, without particular limitationsas long as the relative position between the index patterns and theindex photodiodes are arranged as shown in FIGS. 3A to 3E or FIG. 6 .

It should be mentioned that although the encoding medium 100 isdescribed in the way by moving from right to left as shown in FIGS. 3Ato 3E, the present disclosure is not limited thereto. In other aspects,the encoding medium 100 moves from left to right, and the signalprocessing circuit also generates an index signal pulse at the eventshown in FIG. 3C.

As mentioned above, the conventional optical encoder is not able todistinguish noises caused by the contamination on the encoding mediumsuch that incorrect index output can be generated. Accordingly, thepresent disclosure further provides an encoding medium of an opticalencoder (e.g., FIG. 1 and FIG. 6 ) and a signal processing circuit of anoptical encoder (FIGS. 4A-4B and FIGS. 7A-7B) that identify whether theencoding medium is moved or rotated to a predetermined positionaccording to multiple sets of comparison signals so as to increase thenoise reduction ability of the optical encoder.

Although the disclosure has been explained in relation to its preferredembodiment, it is not used to limit the disclosure. It is to beunderstood that many other possible modifications and variations can bemade by those skilled in the art without departing from the spirit andscope of the disclosure as hereinafter claimed.

What is claimed is:
 1. A signal processing circuit of an opticalencoder, the optical encoder comprising a first index photodiode, asecond index photodiode and a reference index photodiode, the signalprocessing circuit comprising: a trans-impedance amplifier (TIA),electrically coupled to the first index photodiode, the second indexphotodiode and the reference index photodiode, and configured to converta first photocurrent generated by the first index photodiode, a secondphotocurrent generated by the second index photodiode and a referencephotocurrent generated by the reference index photodiode respectively toa first voltage, a second voltage and a reference voltage; a firstcomparator, configured to compare the first voltage and the referencevoltage to output a first comparison voltage; a second comparator,configured to compare the second voltage and the reference voltage tooutput a second comparison voltage; and an AND gate, configured toreceive the first comparison voltage and the second comparison voltageto accordingly generate an index output.
 2. The signal processingcircuit as claimed in claim 1, wherein the TIA comprises: a first TIA,comprising a first input end and a second input end, wherein the firstinput end is configured to receive the first photocurrent to generatethe first voltage, and the second input end is configured to receive thereference photocurrent to generate the reference voltage; and a secondTIA, comprising a third input end and a fourth input end, wherein thethird input end is configured to receive the second photocurrent togenerate the second voltage, and the fourth input end is configured toreceive the reference photocurrent to generate the reference voltage. 3.The signal processing circuit as claimed in claim 1, wherein when thefirst photocurrent and the second photocurrent are larger than thereference photocurrent, the index output has a potential change toindicate a predetermined position of an encoding medium of the opticalencoder.
 4. The signal processing circuit as claimed in claim 3, whereinthe potential change forms a voltage pulse.
 5. The signal processingcircuit as claimed in claim 1, wherein the optical encoder furthercomprises a third index photodiode, the TIA is further electricallycoupled to the third index photodiode and configured to convert a thirdphotocurrent generated by the third index photodiode to a third voltage,the signal processing circuit further comprises a third comparatorconfigured to compare the third voltage and the reference voltage tooutput a third comparison voltage, and the AND gate is configured toreceive the first comparison voltage, the second comparison voltage andthe third comparison voltage to accordingly generate the index output.6. The signal processing circuit as claimed in claim 5, wherein the TIAcomprises: a first TIA, comprising a first input end and a second inputend, wherein the first input end is configured to receive the firstphotocurrent to generate the first voltage, and the second input end isconfigured to receive the reference photocurrent to generate thereference voltage; a second TIA, comprising a third input end and afourth input end, wherein the third input end is configured to receivethe second photocurrent to generate the second voltage, and the fourthinput end is configured to receive the reference photocurrent togenerate the reference voltage; and a third TIA, comprising a fifthinput end and a sixth input end, wherein the fifth input end isconfigured to receive the third photocurrent to generate the thirdvoltage, and the sixth input end is configured to receive the referencephotocurrent to generate the reference voltage.
 7. The signal processingcircuit as claimed in claim 5, wherein when the first photocurrent, thesecond photocurrent and the third photocurrent are larger than thereference photocurrent, the index output has a potential change toindicate a predetermined position of an encoding medium of the opticalencoder.
 8. The signal processing circuit as claimed in claim 7, whereinthe potential change forms a voltage pulse.
 9. The signal processingcircuit as claimed in claim 1, wherein the optical encoder comprises anencoding medium having an index track thereon configured to indicate apredetermined position of the encoding medium, and the index trackcomprises a first index pattern and a second index pattern arranged in atransverse direction, and when the first index pattern is aligned withthe first index photodiode and the second index pattern is aligned withthe second index photodiode, the reference index photodiode is notaligned with the first index pattern or the second index pattern. 10.The signal processing circuit as claimed in claim 9, wherein thereference index photodiode is between the first index photodiode and thesecond index photodiode in the transverse direction.